A national historic preservation organization has slammed proposed changes to the Presidio's Main Post and Parade Ground, including the addition of a 100,000-square-foot modern art museum, according to a new letter sent to the executive director of the Presidio Trust.

The May 16 letter by Anthony Veercamp of the National Trust for Historic Preservation questioned whether the Presidio's designation as a national historic landmark could be threatened by the scale and contemporary design of the museum, as well as other proposed projects. Other concerns include the proposed construction of a three-story lodge, the letter states.

Many of Veercamp's comments echo issues raised last month in a letter by Brian O'Neill, general superintendent of the National Park Service's Golden Gate National Recreation Area. Both the Park Service and the nonprofit National Trust for Historic Preservation are charged with advising the Presidio Trust to ensure that any changes to the Presidio do not hurt its standing as a national historic landmark district.

A spokeswoman for the Presidio Trust, which was created by Congress to manage the 232-year-old military post, said the preservationists' concerns are premature, noting that the trust has just begun the process of assessing the proposed development.

In January, the Presidio Trust chose a proposal by Gap founders Doris and Don Fisher to construct a 100,000-square-foot museum in the center of the Presidio. The museum is the largest of a handful of projects proposed for the area. Currently, the trust is in the process of studying the environmental impacts of the proposed construction. A version of that environmental report will be released next month to allow for public comment before the document is finalized.